This invention relates to the production of glycine by the reductive amination of glyoxylic acid according to the following reaction: EQU OHC .multidot. COOH + NH.sub.3 + H.sub.2 .fwdarw. NH.sub.2 CH.sub.2 COOH + H.sub.2 O
glycine (glycocoll; aminoacetic acid) is an industrial chemical having important and varied uses. Because of its amphoteric nature, it is used widely as a buffering agent, particularly in pharmaceutical and cosmetic preparations. It also has extensive application as a food-grade antioxidant, as a corrosion inhibitor, in electroplating, as an additive to saccharin for preventing its bitter after taste, and in the manufacture of plastics and polymers.
At the present time glycine is manufactured commercially from ammonia and glycolonitrile by a procedure outlined in U.S. Pat. No. 3,813,434. However, this method is complex; involves the use of a toxic reagent (HCN); and leads to the production of glycine in a reaction mixture from which it is isolated in the pure condition only with difficulty.
It has been proposed to produce glycine directly from glyoxylic acid by the reductive amination of the latter in aqueous solution using a palladium catalyst (Desnuelle et al. Bull. Soc. Chim 5! 1, 700-2 (1934); Chemical Abstracts Volume 28 column 6,700 1934). The proposed procedure has the additional advantage of employing as a starting material glyoxylic acid, which is readily available at low cost on the large commercial scale as a product of the controlled, oxidative degradation of cellulose, and especially of paper-making pulps and sludges. (Hearon et al. U.S. patent application Ser. No. 628,888, filed Nov. 5, 1975, now U.S. Pat. No. 3,998,878, for SELECTIVELY SEPARATING OXALIC, TARTARIC, GLYOXYLIC AND ERYTHRONIC ACIDS FROM AQUEOUS SOLUTIONS CONTAINING THE SAME.)
However, the procedure for the reductive amination of glyoxylic acid reported by Desnuelle et al. supra, indicates little promise for the successful commercial application of the procedure, since the glycine product is obtained in a yield of only 8% of the theoretical, and can be isolated from the complex reaction mixture only in the form of a derivative, i.e., the beta-naphthylsulfonate.
The reason for the lack of success in executing the proposed synthetic procedure is evident when it is considered that numerous side reactions leading to the production of numerous byproducts can occur when it is attempted to produce glycine by treating glyoxylic acid with ammonia in an atmosphere of hydrogen and in the presence of a hydrogenation catalyst. Among these are:
The reduction of glyoxylic acid to glycolic acid.
The dimerization and trimerization of glyoxylic acid.
The autooxidation and reduction of the glyoxylic acid in alkaline medium by the well known Cannizzaro reaction to produce oxalic acid and glycolic acid.
The reaction of any of the foregoing acids with ammonia to form ammonium salts of varying degrees of solubility in the reaction medium.
We now have discovered, and it is the essence of the present invention, that the foregoing problems may be overcome and glycine produced directly from glyoxylic acid in yields of up to about 97% by weight by reductive amination carried out with a colloidal rhodium catalyst in a reaction medium comprising a mixture of water and selected water-soluble solvents employed in amounts predetermined to maintain the reactants and reaction products in solution, and to inhibit the occurrence of undesirable side reactions, in particular the Cannizzaro reaction.
In its broad aspect, the hereindescribed process comprises forming a mixture of glyoxylic acid, ammonia and water together with a selected water-soluble organic solvent used in amount sufficient to insure the solubility of the intermediate reaction products during the progress of the reaction and the separation of the desired glycine product at its conclusion.
The reaction mixture is subjected to the action of gaseous hydrogen in the presence of a colloidal rhodium hydrogenation catalyst at pressures ranging from substantially atmospheric pressure to pressures of the order of 3,000 pounds per square inch, and at temperatures varying from just above the freezing point of the reaction mixture to 40.degree. C. The reaction is permitted to proceed until the theoretical amount of hydrogen has been absorbed by the system, which occurs usually in from 1 to 16 hours. At the conclusion of the reaction, the catalyst is removed by filtration, the filtrate concentrated, and a suitable precipitating solvent such as methanol added. Thereupon the glycine separates as a white precipitate which may be separated from the mother liquor by filtration.